1. Field of the Invention
This invention relates to an alkaline developable photocurable material which is useful for the preparation of printing materials, the formation of images, the preparation of printed circuit boards, and the like and, more particularly, to a PS (lithographic printing) plate comprising a light-sensitive layer formed on a sheet or film-like support containing a hydrophilic surface.
2. Discussion of the Prior Art
Various negative light-sensitive materials have been known heretofore. They are generally classified into three types: a first type in which the polymeric compound itself contains a functional group such as an unsaturated bond or an azide group and photocuring is induced directly within the functional polymeric compound or in the optional presence of a photoactivator; and a second type in which the light-sensitive material comprises a mixture of a polymeric compound having no functional group, a normally low molecular weight cross-linking agent having at least two functional groups such as unsaturated groups or azide groups, and a photoactivator, and the non-functional polymeric compound is photocured by the cross-linking agent; and a third type which is combination of the first and second types in which a functional polymeric compound and a cross-linking agent are used together.
When using the first and third types, special consideration must be given to the reproducibility of the quality and properties of the light-sensitive material during manufacture. There are basically two synthetic approaches to a polymer containing a functional group. One approach is to bond a compound containing a functional group (such as an unsaturated bond or an azide moiety) to the reactive cites of a polymer backbone by an addition or condensation reaction or the like. The other is to copolymerize with other monomers, a monomer containing two functional groups of different reactivity, one of which enters into the polymerization reaction to form the polymer chain and the other of which does not react but remains available for photoaction. In both cases, the number of functional groups in the polymer can vary greatly from one batch to the next even if great care is excercised. In the extreme cases, the polymer may gel during reaction or during storage by cross-linking upon itself. Furthermore, a development residue in the form of scum or whisker tends to form during use and the sharpness of the image becomes unsatisfactory. With the second type of material, however, since the polymeric compound is non-functional, there is no restriction on handling such as in purification, and the polymeric compound can be selected from a wide range of materials. Selection of the cross-linking agent can also be performed independently if consideration is given only to its compatibility with the polymeric compound. Thus, it has the general characteristic that a composition excellent in the sharpness of the image and storability is easy to obtain.
Typical examples of the second type composition include a copolymer of methacrylic acid and methyl methacrylate (mole ratio 1:9), a cross-linking agent having ethylenically unsaturated groups and a sensitizer such as an anthraquinone or a combination of a bisimidazole derivative and a diaminobenzoyl derivative (U.S. Pat. Nos. 3,458,311 and 3,549,367). Systems comprising cellulose acetate butyrate, cellulose acetate and a cross-linking agent containing ethylenically unsaturated groups, and a photoactivator, etc., are also known and disclosed in U.S. Pat. No. 3,661,588.
When using the second type composition mentioned above, however, it is necessary to provide a protective layer or film, which is non-sensitive and transparent to actinic light and has low oxygen permeability, on the coated layer. Tracing experiments show that without this type of protective layer, adequate sensitivity and resolution cannot be obtained. In the above-mentioned example, photocuring by the formation of free radicals of ethylenically unsaturated groups is markedly interfered by oxygen. In this regard, with the first type of material, the functional group is introduced directly into the polymer and the affect of oxygen in the air is negligible in practical application.
According to known methods, oxygen-protecting layers are prepared by casting a solution of a water-soluble polymeric compound such as polyvinyl alcohol or polyvinyl pyrrolidone on a light-sensitive layer. Alternatively, the light-sensitive layer is covered with a thin film of polyethylene terephthalate or polypropylene having a thickness of about 25.mu.. The water-soluble polymeric protective layer is removed simultaneously at the time of development with weak alkali. The insoluble thermoplastic protective film is peeled off before development.
Normally, the compound used as the cross-linking agent is a viscous liquid at room temperature. To obtain desirable sensitivities, it is necessary to add the cross-linking agent to such an extent that the light-sensitive layer becomes considerably tacky. Thus, when an original pattern is to be contacted intimately with the light-sensitive layer for exposure, the tackiness of the light-sensitive layer becomes an obstacle without a protective layer, resulting in damaging the original. For this reason, the presence of a protective layer is essential in the known methods.
Usually, a solution of the light-sensitive material in a low boiling organic solvent is cast on the surface of a substrate, and dried under heat to form a light-sensitive layer. It is very complicated from the viewpoint of process steps to cast a solution of a water-soluble polymeric compound on the light-sensitive layer, and dry it under heat to remove water and form a protective layer. The use of a protective film requires a step of laminating the film to the light-sensitive layer. Moreover, in a system having a protective film, it is difficult in practice to resolve a pattern having a smaller line width than the thickness of the film. Scattering of actinic light at the boundary surface between the light-sensitive layer and the protective layer also leads to a deterioration in the sharpness of the image.
Thus, using the second type composition, conventional methods suffers from disadvantages and restrictions because it essentially requires the presence of a protective layer.